1. Field of the Invention
The present invention relates to a fixing device and a fixing roller of an image forming apparatus such as a laser printer, a copier, and the like.
2. Description of the Related Art
As a fixing device of an image forming apparatus such as a laser printer, a copier, and the like, a heat roller fixing system has been known, in which recording material (for example, paper, an OHP sheet, or the like) holding an unfixed toner image is nipped and transported while the recording material being heated and pressed by a pair of fixing rollers, thereby to fix the unfixed toner image onto the recording material. Generally, a pair of the fixing rollers comprises a heat roller having a heat source, and a pressure roller not having the heat source, provided so as to be brought into pressure contact with the heat roller. However, in a high-speed printing machine, or a field of a printing machine in which unfixed toner images formed on both sides of the recording material are simultaneously fixed, it is proposed that the fixing roller comprises a pair of the heat rollers.
In the heat roller fixing system of this kind, at least one roller of a pair of the heat rollers is composed of a drive roller, and the other is composed of a driven roller that is rotated by the pressure contact with the drive roller. One mode of power transmission mechanism in this case has been disclosed in JP-B-6-93154 and JP-B-7-46253, in which a key groove formed at a roller end portion is fitted to a drive key provided for a drive force-transmitting shaft, thereby to transmit the drive force to the fixing rollers.
In the heat roller fixing system disclosed in JP-B-6-93154 and JP-B-7-46253, the heat roller forming the fixing roller pair is constituted as a drive roller; and the diameter of the heat roller is set to 100 mm, the total length thereof (in the axial direction) is set to 560 mm, and the average thickness of the core metal material (for example, made of aluminum) thereof is set to 8 mm, thereby to obtain a large caliber heat roller and rotate this heat roller at the number of revolutions of 216 rpm.
As a result, a moment of inertia in a direction of a rotational shaft of the heat roller reaches about 70 Kgxc2x7cm2 In case that the key groove (in the key groove, buffer material as described in JP-B-7-46253 is provided) of the heat roller has received the drive force from the drive key for a long time, before the print amount reaches the permissible amount, break was produced due to heat and fatigue. Further, since the heat capacity of the core metal material of the heat roller was also large (about 2 kW), consumed power at the above number of revolutions was also large.
Accordingly, under the above constitution, the break of the key groove or of the buffer material provided in the key groove proceeds more speedily than in the usual driving system. Therefore, even if the fixing roller does not reach an end of lifetime, such disadvantage is caused that the expensive fixing roller must be exchanged due to the break of the key groove or of the buffer material.
An object of the invention is, even in case that the fixing roller is rotated at a high speed, without causing the increase of the consumed electric power, to provide a fixing device and a fixing roller of an image forming apparatus which can suppress increase of the moment of inertia in the direction of the rotational shaft of the fixing roller and early break of the key groove, and which can lengthen particularly the lifetime of the fixing roller.
In order to achieve the above object, a fixing device of an image forming apparatus comprises:
a first fixing roller;
a second fixing roller come into pressure contact with the first fixing roller, the second fixing roller including a cylindrical metal core having an outer diameter of 80 to 120 mm and a length in axial direction of 470 to 600 mm, the second fixing roller defining a first groove at one end of the cylindrical metal core; and
a driving shaft having a protrution engaged with the first groove of the cylindrical metal core; wherein the second roller is driven and rotated at the number of revolutions of 60 to 300 rpm; and an average thickness of the cylindrical metal core is regulated so that the moment of inertia in a direction of a rotational shaft of the second fixing roller is no more than 70 Kgxc2x7cm2.